Radios operating in the extremely high frequency (EHF) band of the electromagnetic (EM) spectrum exhibit numerous advantages, and are anticipated to play a significant role in communication technology—particularly wireless, mobile communication devices. For example, radios operating in EHF exhibit numerous advantages over radios operating in other frequency bands of the spectrum, including license-free spectrum, relatively narrow beam antennas, and inherent security due to oxygen absorption and the narrow beam width.
As used herein, the extremely high frequency (EHF) band of the EM spectrum includes frequencies from approximately 30 to 300 GHz. This is the highest frequency range of what is considered to be Radio Frequency (RF) EM radiation. Above this frequency band, EM radiation is considered to be in the low infrared light spectrum (also referred to as terahertz radiation). EM energy in the EHF band has a wavelength in the range of approximately 10 mm to 1 mm. Hence, EHF EM radiation is also generally referred to as millimeter wave RF (mm-wave). Accordingly, the terms EHF and mm-wave are used synonymously herein when referring to a frequency band.
In the U.S., the Federal Communication Commission (FCC) has allocated an unprecedented 7 GHz of un-channelized spectrum for license-free operation between 57-64 GHz. In contrast, less than 0.5 GHz of spectrum is allocated between 2-6 GHz for WiFi and other license-free applications. The portion of the EHF band near 60 GHz thus represents a significant opportunity to implement multi-gigabit RF communication links. Standardization efforts in this area include WiGig and WirelessHD.
EHF radios utilize very narrow RF beams, enabling multiple EHF radio base stations or other transceivers to be installed on the same tower, rooftop, or the like, even if they are all operating at the same transmit and receive frequencies. Co-located radios operating in the same transmit and receive frequency ranges can easily be isolated from one another based on small lateral or angular antenna separations, and/or the use of cross-polarized antennas. While the RF beams are relatively narrow, however, they are sufficiently wide, e.g., compared to optical signals, such that fixed antennas may be accurately aligned by a non-expert installer with the use of a simple visual alignment tool, and communications are unaffected by minor antenna movement such as tower or building sway due to wind.
Oxygen attenuates RF signals near 60 GHz (e.g., ˜57-64 GHz) due to a resonance of the oxygen molecule, a property that is unique to the near-60 GHz portion of the EM spectrum. While this property limits the distances that radio links at this frequency can cover, it also makes these links highly immune to interference from other radios at the same or near frequencies. For example, oxygen absorption ensures that a near-60 GHz signal will not extend far beyond its intended target.
The combination of narrow beam width and oxygen attenuation provides an inherent degree of security to near-60 GHz link communications. Due to the narrow beam width, an interceptor receiver must be placed directly in the main beam (and tuned to its carrier frequency) to receive a useful signal. In this position, it is likely to degrade the signal at the intended receiver sufficiently to allow for its detection. Due to oxygen attenuation, there is a limited distance beyond an intended receiver, along the main beam, at which a useful signal may be obtained by an interceptor receiver.
Accordingly, the demand is increasing for EHF capability in mobile communication devices, particularly near 60 GHz, to allow them to engage in communication channels supplemental to their primary channels (e.g., GSM, CDMA, LTE, and similar systems). However, high frequency electronics consume significant amounts of power, and hence are a major factor in depleting useful battery life. In particular, the millimeter-wave power amplifier (PA) is the most power-hungry block in an EHF transceiver. A typical requirement of an EHF PA is to deliver at least 10 dBm output power to set up a communication range of 1 m.
A conventional approach to satisfying both the high output power demands of EHF radios and minimizing power consumption (and hence battery depletion) when not transmitting, is a dual-mode PA in which two or more unit PAs are coupled together to achieve high output power. When not transmitting, one or more of the PA units may be disabled to reduce power consumption. The output signals of the units are typically combined with a transformer-based combiner. Such a combiner achieves an insertion loss of as little as 1.2 dB. However, the combiner loss can be as high as 5 dB in low-power mode, due to the extra loss introduced by the parasitic loading of the unit PA(s) in an off state.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.